A compartmental tissue culture system has been used to control neuronal activity and to measure the strength of synaptic connections between defined neuronal and target populations. Abundant synaptic connections develop between cholinergic neurons (from the superior cervical ganglion or the ventral spinal cord) and muscle cells. Electrical activation of the cholinergic synapase induces a substantial but non-selective elimination of the connections. Motile growth cones of electrically inactive dorsal root ganglion (DRG) neurons are normally associated with low levels of intracellular calcium [Ca++]i (less than 200 nM). Stimulation of these neurons produces a transient increase in [Ca++]i, (up to 1000 mM) and a cessation of growth. Continued stimulation for several hours results in a lower [Ca++]i, which, however, is higher than that of unstimulated axons. Resumed axonal growth is seen in this condition. Mechanisms (including decreased Ca++ channel activity and increased calcium extrusion or sequestration are being investigated for their role in both adjustment of [Ca++]i and axonal growth. The 43 kD growth associated protein GAP-43 is developmentally regulated, being most abundant in young cultures and declining between the second and third week in culture. Electrical stimulation of DRG neurons for 3-5 days results in an increase in GAP-43 levels. Because of its location in presynaptic structures and its cell biologic functions, GAP-43 modulation may well be involved in activity-dependent synaptic plasticity.